The invention relates to an optical fiber that is employed in an optical component used for optical communications, to an optical fiber coupler, to an erbium-doped optical fiber amplifier, and to an optical waveguide using the same. More specifically, the invention relates to an optical fiber that can couple with another optical fiber having a different mode field diameter (MFD) with low splice loss and a sufficient splicing strength when fusion-spliced together, to an optical fiber coupler, to an erbium-doped optical fiber amplifier, and to an optical waveguide using the same.